| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel sheet for a low yield ratio high strength steel which can stably obtain a steel pipe having a yield stress YS of≥650 MPa and a yield ratio YR of≤90%, to provide a method for producing the same, and to provide a low yield ratio high strength steel pipe obtained by subjecting the steel sheet to warm forming. SOLUTION: Disclosed is a steel sheet for a low yield ratio high strength steel pipe having a composition comprising, by mass, 0.03 to 0.10% C, 0.15 to 0.50% Si and 1.4 to 3.0% Mn, further comprising one or two kinds selected from 0.1 to 1.0% Cu and 0.1 to 2.0% Ni, and further comprising one or more kinds selected from 0.1 to 1.0% Cr, 0.1 to 1.0% Mo and 0.01 to 0.1% V, and having a microstructure composed of a mixed structure consisting of bainite having a Vickers hardness Hv of 180 to 280 and insular martensite having a volume fraction of 5 to 20% and a Vickers hardness Hv of 500 to 650. COPYRIGHT: (C)2009,JPO&INPIT
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